The use of dielectric materials to increase capacitance is well-known. Earlier capacitor dielectrics fell into two categories. The first category of dielectrics has a relatively temperature-independent dielectric constant but the value of the dielectric constant is low, e.g., 5–10. Materials such as electrical porcelain and mica fall in this category. The second category of dielectrics has very high dielectric constant, e.g., 1000 or more, but they are quite temperature dependent. An example is barium titanate, BaTiO3.
Since capacitance of a dielectric material is proportional to its dielectric constant, high dielectric constant materials are desired. In order to perform acceptably in tuning or resonance circuits, the dielectric must also have a dielectric constant that exhibits minimal temperature dependence; otherwise, small changes in ambient temperature will throw the circuit out of resonance. Other applications require a dielectric constant that exhibits minimal frequency dependence. It is also desirable to have the loss or dissipation factor as small as possible.
For many microwave devices, the important material features are the dielectric tunability, i.e., the change in dielectric constant with applied voltage, and low dielectric loss. Barium strontium titanate, Ba1−xSrxTiO3, has been used in some such applications (see U.S. Pat. No. 5,427,998), but the need persists for materials with better properties.
Ismailzade et al., Phys. Stat. Sol. (a), 59, K191 (1980), studies the ferroelectric transitions of the system (1−x)BaTiO3−xBiFeO3 (x=0.05 and 0.08) and reported transition temperatures of 106° C. and 85° C., respectively.
Hagemann et al., J. Amer. Ceramic Soc., 64 (10) 590 (1981), studied the defect chemistry of BaTiO3 doped with acceptors Cr, Mn, Fe, Co and Ni. They note that conventional titanate formulations usually contain donor dopants like Nb or La, requiring that sintering and all subsequent high temperature treatments be performed at high partial pressure of oxygen.
Inoue et al., Jpn. J. Appl. Phys., 30 (9B), 2388 (1991), measured the dielectric constant and losses in BaTiO3 ceramics doped with acceptors Fe, Ni, Cu and Nb.
Skapin et al., J. Solid State Chem., 129, 223 (1997) found that the solid (Ba,La)(Ti,Al)O3 exhibits relatively low permittivities and very low dielectric losses.
Sengupta et al., Mat. Res. Innovat., 2, 278 (1999), investigate the effects of acceptor and donor doping on the electronic properties of barium strontium titanate thin films by doping with 1 mole % of Mg, La, Ta and Al. They state that MgO doping produced the best effect on lowering the loss tangents at microwave frequencies while maintaining tunability.
Sengupta et al., U.S. Pat. No. 5,427,988, disclose ceramic ferroelectric composite material consisting essentially of barium strontium titanate, Ba1−xSrxTiO3, wherein x is greater than 0.0 and less than or equal to 0.75, and magnesium oxide, MgO, in amounts to provide a composite having a low dielectric constant, low loss tangent and high tunability. Their preferred weight ratio of Ba1−xSrxTiO3 to MgO ranges from approximately 99%–40% Ba1−xSr3xTiO to 1%–60% MgO.
Yandrofski et al., U.S. Pat. No. 5,472,935, disclose tuneable microwave devices incorporating tuneable ferroelectrics.